As is known in the prior art, the alternator-starter machine comprises a rotor sized so as to be able, in starting mode, to start the thermal engine. An excitation coil of the rotor is supplied with a pulse width modulated (PWM) excitation current having a duty cycle changing between 0 and 100%. In alternator mode, the machine functions with a duty cycle from 0 to 50%. In pre-fluxing mode or in electric motor mode, the machine can be controlled with a duty cycle of 100% so as to obtain a maximum mechanical torque in order to rotate the thermal engine and start it.
Micro-hybrid vehicle systems are known comprising an alternator-starter and an auxiliary starter. For cold starts of the thermal engine, use of either the auxiliary starter alone or the auxiliary starter and the alternator-starter operating in concert is known.
When the auxiliary starter and the alternator-starter operate in concert, in a first phase the auxiliary starter alone commences the driving of the thermal engine and, simultaneously, a pre-fluxing excitation current is supplied to the rotor of the alternator-starter in order to prepare it for functioning in motor mode. In a second phase, if necessary, that is to say in the case where the thermal engine has not started during the first phase, the alternator-starter can participate in the driving of the thermal engine and finalise the starting thereof.
During the pre-fluxing step, the rotor of the machine can be pre-fluxed with a duty cycle of 100% without supplying any torque. In other words, the alternator-starter then functions in generator mode with a duty cycle much greater than 50%, that is to say beyond a maximum excitation generally acceptable in generator mode. There are therefore, in this known micro-hybrid system, risks of generating overvoltages, firstly on an onboard network of the vehicle and secondly at the terminals of an energy storage unit of the system, in particular a battery, this network and battery being supplied electrically by the alternator-starter when the latter functions in generator mode.
Such a situation can also occur in the case of an instruction to restart the thermal engine occurring little after an instruction to stop the said engine, when the latter does not yet have a zero rotation speed. In such a case, the thermal engine being hot, the starting thereof is attempted only with the alternator-starter. The rotor pre-fluxing step is carried out prior to a supply to the stator windings of the machine and the machine functions as a generator during this step where the thermal engine is still turning over because of the inertia thereof. Injection of fuel into the thermal engine being immediately re-established following the instruction to restart it, a starting of the thermal engine hot can occur while the pre-fluxing step is under way with a duty cycle of 100%, causing an increase in speed of the machine and risks of generating overvoltages.
The invention aims to remedy the aforementioned drawbacks.